fix: make reasoning and details disclosures keyboard operable (WCAG 2.1.1, 4.1.2) (#27490)

On latest `dev`, the `title !== null` branch of `Collapsible` renders its header as a bare `<div>` whose only handler is `on:pointerup`, with the two Svelte a11y warnings suppressed above it.

`pointerup` is never dispatched by keyboard activation, and the `<div>` has no `role`, no `tabindex` and no `aria-expanded`. The header is therefore not focusable, not activatable and not announced as a control. This is the header for "Thinking..." / "Thought for N seconds", "Analyzing..." / "Analyzed", and every `<details>` block rendered from model output, via `Messages/Markdown/MarkdownTokens.svelte`, `Messages/StructuredOutputRenderer.svelte` and `chat/Controls/Controls.svelte`.

In practice a keyboard or screen reader user cannot expand any model reasoning trace, tool call detail or code interpreter block, and a screen reader reads the header as static text with no hint that anything is collapsed behind it.

Breaks WCAG 2.1.1 Keyboard (Level A), since the disclosure has no keyboard operation at all, and 4.1.2 Name, Role, Value (Level A), since it exposes neither a button role nor its expanded state.

Fix: render that header as a real `<button type="button">` with `aria-expanded` and the native `disabled` attribute, and toggle on `click`, which fires for both pointer and keyboard activation. This branch contains no `<slot />` and no interactive descendants, so a button is valid here. `block text-start` keeps the previous box and alignment behaviour, since a `<button>` otherwise defaults to `inline-block` and centred text. `disabled:cursor-default` replaces the old `{disabled ? '' : 'cursor-pointer'}` ternary, which became a no-op once this was a button, because `src/tailwind.css` applies `cursor-pointer` to every `button`. Verified in a browser that display, text alignment and rendered height match the previous `<div>`, and that a disabled header no longer shows a pointer cursor.

Switching from `pointerup` to `click` also means the header no longer toggles on right click, or when a drag starts outside it and ends inside.

The `{:else}` branch is deliberately left alone. Its `<slot />` receives buttons from `Sidebar/Section.svelte`, `common/Folder.svelte` and `Sidebar/RecursiveFolder.svelte`, so it cannot legally become a `<button>` and needs a different fix.

Severity: Critical. Model reasoning output is entirely unreachable without a mouse.

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src/lib/components/common/Collapsible.svelte

@@ -71,9 +71,13 @@
 
 <div {id} class={className}>
 	{#if title !== null}
-		<!-- svelte-ignore a11y-no-static-element-interactions -->
-		<!-- svelte-ignore a11y-click-events-have-key-events -->
-		<div class="{buttonClassName} {disabled ? '' : 'cursor-pointer'}" on:pointerup={toggleOpen}>
+		<button
+			type="button"
+			class="{buttonClassName} block text-start disabled:cursor-default"
+			aria-expanded={open}
+			{disabled}
+			on:click={toggleOpen}
+		>
 			<div
 				class=" w-full flex items-center justify-between gap-2 {attributes?.done &&
 				attributes?.done !== 'true' &&
@@ -128,7 +132,7 @@
 					</div>
 				{/if}
 			</div>
-		</div>
+		</button>
 	{:else}
 		<!-- svelte-ignore a11y-no-static-element-interactions -->
 		<!-- svelte-ignore a11y-click-events-have-key-events -->